1. Field of the Invention
This invention relates to a solid electrolytic capacitor, such as tantalum capacitor or aluminum capacitor, which incorporates a safety fuse wire in addition to a capacitor element.
2. Description of the Related Art
A solid electrolytic capacitor (e.g. tantalum capacitor) is known to generate a large amount of heat upon passage of an overcurrent which might be caused by an insulation failure for example. Further, since the solid electrolytic capacitor is a polar element, it also generate a lot of heat if mounted on a circuit board with reverse polarity.
To prevent such heat generation, therefore, it has been proposed to incorporate a built-in safety fuse wire in a resin package of the solid electrolytic capacitor, as disclosed in Japanese Patent Application Laid-open No. 2(1990)-105513 for example. For the convenience of description, such a capacitor is illustrated in FIGS. 6 through 8 of the accompanying drawings.
As shown in FIG. 6, a prior art fused solid electrolytic capacitor comprises a capacitor element 1 which includes a capacitor chip 2 and an anode wire 3. The capacitor chip 2 has a first end face 2a from which the anode wire 3 projects out, a second end face 2b opposite to the first end face 2b, and an upper side face 2c between the first and second end faces 2a, 2b. The anode wire 3 is electrically connected to an anode lead 4 by welding, whereas the side face 2c of the chip 2 is electrically connected to a cathode lead 5 through a safety fuse wire 6 partially enclosed in a relatively soft elastic resin member 7. All of these components are enclosed in a package 8 of a synthetic resin with the respective leads 4, 5 partially projecting therefrom, and the projecting portions of the respective leads 4, 5 are bent toward the underside of the package 8 for conveniently mounting to a surface of a printed circuit board (not shown).
According to the arrangement described above, the fuse wire 6 is bonded to the upper side face 2c of the chip 2 at a position close to the second end face 2b thereof. Thus, the cathode lead 5 need be sufficiently spaced from the second end face 2b of the chip 2 to make sure that the fuse wire 6 has a sufficient length S as required for providing an intended breaking (melting) property. As a result, the length L' of the capacitor must be inevitably increased.
Further, due to the large spacing between the chip 2 and the cathode lead 5, it becomes difficult to locate an intermediate portion of the fuse wire 6 close to the chip 2. Thus, when used as a temperature fuse, the fuse wire 6 cannot be made sufficiently sensitive to the heat of the chip 2.
The above-described problems of the prior art solid electrolytic capacitor can be reduced by bonding one end of the fuse wire 6 to the upper side face 2c of the chip 2 at a position reasonably spaced from the second end face 2b of the chip 2 and by making the fuse wire 6 to extend along an upwardly curved path with a vertical spacing W' from the upper side face 2c of the chip 2, as shown in FIGS. 7 and 8. However, this solution gives rise to the following new problems.
Before partially enclosing the fuse wire 6 in the soft elastic resin member 7 followed by subsequent formation of the resin package 8, the fuse wire 6 may gravitationally sag down toward the upper side face 2c of the chip 2, as indicated by phantom lines in FIG. 8. In an extreme case, the fuse wire 6 may come into direct contact with the chip 2.
When used as a temperature fuse wire, since the melting property of the fuse wire 6 varies depending on the spacing from the chip 2, the sagging of the fuse wire results in a variation of the melting property. When used as an overcurrent fuse, on the other hand, the effective length of the fuse wire 6 greatly reduces if the fuse wire 6 sags into direct contact with the chip 2, consequently failing to provide an intended fusing function.
It is conceivable to increase the initial spacing W' between the chip 2 and the fuse wire 6 for avoiding direct contact between the two even upon sagging. However, this countermeasure results in a corresponding increase of the thickness dimension R' of the package portion above the upper side face 2c of the chip 2, so that the height H' of the resin package 8 must also be increased.